On Tuesday, August 18, 1942, Train Da 503 pulled away from Platform 40 of Frankfurt’s Grossmarkthalle. Its destination was a small town north of Prague, near an old fortress called Theresienstadt. The train was packed with more than a thousand elderly Jews. Among them were my grandfather Otto Kupfer, 68, and his 72-year-old sister Mina. Both would be dead by the end of the year.